I. Cytokines and the Acute Phase Response
Infection, injury, trauma, and a variety of other immunological disorders provoke a basic immune system defense response known as the "acute phase response." (Van Snick, J., Annu. Rev. Immunol. 8:253-278 (1990); Akira, S. et al., Immunol. Rev. 127:25-50 (1992); Koj, A. In: The Acute Phase Response to Injury and Infection, Elsevier, Amsterdam, vol. 10, p139 (1985)). The acute phase response is part of the general inflammatory host defense mechanism. The acute phase response is characterized by fever, leukocytosis, negative nitrogen balance, increased vascular permeability, alterations in plasma meta and steroid concentrations, and by an increase in the synthesis of hepatic acute phase proteins (such as .alpha..sub.1 -antitrypsin, .alpha..sub.1 -antichymotrypsin, and other protease inhibitors, serum amyloid A, and C-reactive proteins) (Van Snick, J., Annu. Rev. Immunol. 8:253-278 (1990)).
Various proteins (termed "cytokines") are involved in mediating the acute phase response. These cytokines include tumor necrosis factor ("TNF"), transforming growth factor-.beta. ("TGF-.beta."), Interleukin-1 ("IL-1"), Interleukin-2 ("IL-2"), Interleukin-8 ("IL-8"), Interleukin-10 ("IL-10") and Interleukin-6 ("IL-6") (Bauer, J. et al., Ann. Hematol. 62:203-210 (1991); Kishimoto, T., Blood 74:1-10 (1989); Akira, S. et al., Immunol. Rev. 127:25-50 (1992); Van Snick, J., Annu. Rev. Immunol. 8:253-278 (1990)).
IL-6 plays a central role in inducing the acute phase response. Its release produces multiple effects. IL-6 induces B cells to synthesize immunoglobulins; it induces fever, promotes the synthesis of corticotropin by the pituitary, stimulates hepatocyte production of the acute phase proteins, and acts as a growth promoter of mesangial cells and keratinocytes. IL-6 exerts its control on acute phase proteins at least in part at the transcriptional level (Morrone, G. et al., J. Biol. Chem 263:12554-12558 (1988)). IL-6 also is believed to play a role in inducing the proliferation of hematopoietic cells, and particularly, cytotoxic T cells.
IL-6 is produced by a large number of cell types, including fibroblasts, endothelial cells, keratinocytes, monocytes-macrophages, T-cells, mast cells, and a variety of tumor cell lines (Van Snick, J., Annu. Rev. Immunol. 8:253-278 (1990); Bauer, J. et al., Ann. Hematol. 62:203-210 (1991)). Accessory cells appear to produce the major source of IL-6, however, significant amounts of IL-6 are also produced by lymphocytes (Hirano, T. et al., Eur. J. Immunol. 18:1797-1801 (1988)).
IL-6 is a protein of 21-28 kD which exhibits extensive post-translational modification. cDNA encoding IL-6 has been cloned, and predicts a precursor protein of 212 amino acids including a hydrophobic signal sequence of 28 residues (Hirano, T. et al., Nature 324:73-76 (1986)). Recombinant human IL-6 can be obtained from Genzyme Corp., Boston, Mass.
IL-6 is secreted into the serum. Normal serum levels of IL-6 are less than 5 pg/ml (Nachbaur, D. M. et al, Ann. Hematol. 62:54-58 (1991)). The protein is not generally produced constitutively by normal cells (Akira, S. et al., Immunol. Rev. 127:25-50 (1992)). Indeed, constitutive expression is a characteristic of a number of pathologic conditions (such as psoriasis, rheumatoid arthritis, cardiac myxoma, multiple myeloma, Castleman's disease, and HIV infection. The level of IL-6 is regulated by positive or negative stimuli. For example, liposaccharides induce cells to produce IL-6; the secretion of glucocorticoids represses IL-6 expression (Akira, S. et al., Immunol. Rev. 127:25-50 (1992)). Other positive inducers of IL-6 production include viruses, interleukin-1 (IL-1), interleukin-3 (IL-3), granulocyte/macrophage-colony stimulating factor (GM-CSF), tumor necrosis factor (TNF), .beta.-interferon, and platelet-derived growth factor. IL-6 production is induced during acute inflammatory processes, and is produced by cells that have been injured.
IL-6 induction rapidly follows injury or trauma. Plasma levels of IL-6 can be detected as early as 30 minutes after incision in patient's undergoing elective surgery (Shenkin, A. et al., Lymphok. Res. 8:123-127 (1989)). Maximal levels of IL-6 are found between 90 minutes and 6 hours post surgery (Pullicino, E. A. et al., Lymphok. Res. 9:2-6 (1990); Shenkin, A. et al., Lymphok. Res. 8:123-127 (1989)). In contrast, upon exposure to an infectious agent, elevated plasma levels may persist for days (Bauer, J. et al., Ann. Hematol. 62:203-210 (1991)). Maximal IL-6 plasma concentrations after sterile trauma, such as elective surgery, are about 100 pg/ml, which is orders of magnitude less than the level (up to 500 ng/ml) associated with bacterial infection (Fiedler, W. et al., Leukemia 4:462-465 (1990); Helfgott, D. C. et al., J. Immunol. 142:948-953 (1989)). Elevated serum levels of IL-6 have been observed in transplant rejection, and inflammatory bowel disease (van Oers, M. H. J. et al., Clin. Exper. Immunol. 71:314-319 (1988); Bauer, J. et al., Ann. Hematol. 62:203-210 (1991)).